That being said, mantle isnt the boom I thought it would be considering how long it’s been around now (and more complex readability-wise than our standard main chains one could argue) — so it’s not unreasonable as I said to have such a concern as it’s not something that I feel confident “will never happen” the way you described. (See below pls)
Finishing on this:
To OP; I will say the data for Mantle available suggesting <100 users the last 7d isn’t a great sign, I do see MNT is open source contract but does include a proxy contract of some sort which BNB does not contain, both are open source
To chefs: to my understanding that proxy contract could be something as small as using what I believe or called diamond functions regarding smart contract calls.
But for the sake of the argument, I think it would be prudent to have a third-party verify what the proxy contract functions for as that would put any doubts to rest if possible. As proxy contracts admittedly are associated unfortunately with bad actor ploys despite good actors use. @TikiTiki I.e our derivatives infrastructure provider (futures platform people) use a proxy contract so it allows funds held during a trade to be entirely dictated by smart contract functionality.
Thus the custodian at no point holds these funds themselves & both parties can find peace of mind that final output of senders txn is defined. TL;DR/ Proxy contracts are definitely useful but your concern is 110% not out of reality
Solution? — is there another chain’s product we provide you can use in the meantime that you feel more comfortable on? FWIW I really doubt the Chefs are going to risk a scandal like this considering the empire/reputation they’ve worked to establish 
Sorry for the rant but hope it’s helpful!